1. Field of the Invention
The field of the invention is that of wavelength division multiplex optical transmission networks. The invention relates to optical switching systems for use in the routing nodes of such networks.
The invention relates more particularly to reconfigurable optical switches which receive via their input ports a plurality of wavelength division multiplex (WDM) signals and individually route the spectral channels (or, to put it more simply, wavelengths) constituting the WDM signals received to respective output ports selected as a function of a command. The invention also relates to switches adapted to perform the converse operation, i.e. to form at their output ports, from received single-wavelength signals, a plurality of WDM signals each consisting of a combination of the received single-wavelength signals selected as a function of a command.
The switches considered here are transparent, i.e. of the type in which the channels remain in the optical domain.
2. Description of the Prior Art
Switches of the above type are advantageously used in the routing nodes as reconfigurable input or output interfaces of electronic switches.
An interface of the above kind is generally called a patch panel in English. The input patch panel spatially separates the WDM channels received from a plurality of input optical fibers and switches all of those channels to respective output ports. The output ports communicate with respective input terminals of an electronic switch via an optical-to-electrical conversion interface consisting of optical receivers, typically photodetectors.
The output patch panel includes input ports that communicate with respective output ports of the electronic switch via an electrical-to-optical conversion interface consisting of single-wavelength optical signal senders, typically modulated laser sources. These signals are formed into WDM signal channels by the output patch panel, which switches the sent single-wavelength signals and combines them in groups to form multiplexes at respective output ports. The output ports are coupled to respective output optical fibers.
For optimum use of the spectral resources in a network it is desirable for the patch panels to provide some flexibility, i.e. for them to be at least partly reconfigurable by means of a simple command. This is because, considering the input patch panel first, it is clear that the maximum number of received channels that can be processed by a node is imposed by the number of input terminals of the electronic switch (or at least the number thereof in service at a given time). This causes the same limitation on the number of channels that may be used simultaneously in all of the fibers of the network on the upstream side of the node. Now, to make good use of these limited resources, it is necessary to be able to modify their distribution between the fibers, i.e. to be able to reduce the number of channels assigned to a fiber on which traffic is decreasing to the benefit of another fiber on which the traffic is tending to become saturated.
This implies that at least one of the receivers that initially receives a certain channel coming from a certain fiber may thereafter receive instead a channel additionally assigned to another fiber and coming therefrom. With a non-reconfigurable patch panel, such redistribution of channels between the fibers is not possible without physical manipulation of elements of the panel by a technician.
There is a similar requirement in respect of the output patch panel. Here the maximum number of channels that can be sent is imposed by the number of active output channels of the electronic switch (which is generally the same as the number of input terminals). As the output fibers generally correspond to different destinations and each carry traffic that can evolve significantly, it is also necessary to be able to modify by means of a simple command the numbers of channels injected into the respective output fibers.
One prior art solution providing the above function is shown diagrammatically in FIG. 1, which relates to the particular case of a routing node coupled to three input optical fibers and three output optical fibers. The diagram shows the elements relating to a routing node that have just been referred to, in succession:                input optical fibers F1-F3 that carry respective input multiplexes WM1-WM3 via input optical amplifiers OA1-OA3;        an input patch panel PP having input ports A1-A3 coupled to respective outputs of the amplifiers OA1-OA3 and output ports Q1, . . . Qi, . . . Qa;        an optical-to-electrical conversion interface RX consisting of optical receivers RX1, . . . RXi, . . . RXa coupled to respective output ports Q1, . . . Qi, . . . Qa of the patch panel PP;        an electronic switch ESW;        an electrical-to-optical conversion interface TX consisting of optical senders TX1, . . . TXi, . . . TXa (in the particular but usual case in which the numbers of receivers and senders are equal);        an output patch panel PP′ having input ports Q′1, . . . Q′i, . . . Q′a coupled to respective outputs of the senders TX1, . . . TXi, . . . TXa and output ports A′1-A′3; and        output optical fibers F′1-F′3 that receive output multiplexes WM′1-WM′3 from the output ports A′1-A′3 via output optical amplifiers OA′1-OA′3 whose inputs are coupled to respective output ports A′1-A′3.        
To provide the required flexibility on the upstream side of the node, the input patch panel PP uses a first stage consisting of optical demultiplexers DM1-DM3 coupled to respective input fibers followed by a space switch XB. With conventional demultiplexers, i.e. demultiplexers each adapted to provide at its outputs the various channels constituting the multiplex that it receives, the maximum flexibility is obtained if the space switch XB is a crossbar switch enabling selective coupling of each output P1, . . . Pi, . . . Pa of the demultiplexer to any of its output ports Q1, . . . Qi, . . . Qa as a function of a command (no reference number). Partial flexibility may also be provided by using a smaller space switch receiving only some of the channels separated by the demultiplexers, the other channels being guided directly to respective dedicated receivers.
In the same way, to provide the required flexibility on the downstream side of the node, the output patch panel PP′ uses a space switch XB′ followed by a second stage consisting of optical couplers M1-M3 coupled to respective output fibers. The maximum flexibility is also obtained if the space switch XB′ is a crossbar switch allowing selective coupling of each sender output from its input ports Q′1, . . . Q′i, . . . Q′a to any one of the inputs P′1, . . . P′i, . . . P′a of the couplers M1-M3. Partial flexibility may also be provided by using a smaller space switch receiving the signals from only some of the senders, the other signals being guided directly to respective couplers.
The drawback of the above solution is its lack of modularity. A network node is adapted to achieve a given nominal switching capacity, but when the network first begins to function not all of that capacity is used. As requirements increase, upgrading is effected by using new wavelengths in the fibers initially used and/or new fibers. The non-modular character of the arrangement makes it obligatory either to install from the outset of operation a space switch rated for the nominal capacity, but therefore overrated for the initial requirement, or to accept a degraded level of flexibility when the capacity increases. On the other hand, it is desirable to adapt the cost and the performance of the above kind of equipment continuously to the commercial use that is made of it.
The invention seeks to remedy this drawback by proposing a new optical switch structure that may be used as reconfigurable input and output patch panels and which is modular so that its performance may evolve in proportion to its cost, the expression “performance” here designating its capacity to process a certain number of channels accepted as input for a certain number of fibers without degrading flexibility, i.e. the ratio of the number of channels that can be reassigned between fibers to the average number of channels conveyed by each fiber.
To this end, the invention uses as a basic building block an optical component known as a wavelength selection switch or a wavelength switch module available from various suppliers. A component of the above kind was described, for example, at the ECOC′2002 conference, Copenhagen, 9 Sep. 2002, document 2.3.1, entitled “The MWS 1×4: A High Performance Wavelength Switching Building Block”, by T. Ducellier et al.
The above component may be used to effect demultiplexing or multiplexing according to the signal propagation direction. In the former case, one port of the component constitutes an input and several other ports constitute outputs. The component switches spectral channels of a multiplex received at its input to respective outputs of the module selectively as a function of their respective wavelengths and as a function of a command signal.
This component therefore has a programmable demultiplexing function supplying at any selected output either a selected channel from the channels of the received multiplex or an output multiplex consisting of a set of channels selected from the channels of the received multiplex. It has the particular feature of being able to process a large number of received channels, but has a low number of outputs, typically four or eight outputs at present. That number may be increased by cascading a plurality of the above components in a tree structure, however. Hereinafter, a component of the above kind, or an assembly of components equivalent to a component having a greater number of outputs, is referred to as a wavelength selection module or, more simply, as a selection module.
The same module may also perform the converse function by exchanging the output and input roles. In this case, it becomes a component having a plurality of inputs and one output and is used to switch spectral channels (i.e. optical signals carried by respective wavelengths) received at respective inputs to the output of the module selectively as a function of the wavelengths of the channels received at the respective inputs and as a function of a command signal.
Of course, the spectral channels switched to the output must have different wavelengths. The module then has a programmable multiplexing function for providing at its output a channel selected from the channels received or an output multiplex consisting of a set of channels selected from the channels received.